Various engine coolant systems have been designed to maintain the temperature of an engine within a desired range.
It is known that the performance of internal combustion engines can be improved by increasing the density of air entering the combustion chamber. Turbochargers and superchargers have been developed to increase the air entering the combustion chambers. However, an undesirable effect of compressing the air to be applied to the combustion chambers is the heating of the air. It is preferred to maintain the temperature of the engine intake air at a specific temperature for maximum engine efficiency. Thus aftercoolers have been developed to cool the compressed air prior to application of the air to the combustion chambers. Typically, the aftercooler is included in the coolant flow loop for the engine block to maintain the temperature of the charged air near the coolant flow temperature.
Often, the desired operating temperature for the engine block, and thus the temperature of the engine coolant, is not the desired temperature for operation of the aftercooler. Tuned, low-flow coolant systems were designed to alleviate this problem. A typical low-flow coolant system is described in SAE Technical Paper Series, 841023, "Selection of the Optimized Aftercooling System for Cummins Premium Diesel Engines," Geoffrey Bond, et al., pp. 4-5, 1984. In the "tuned low-flow system" described in the Series, the engine block is in a relatively high-flow coolant loop wherein approximately 90% of the coolant can flow through this loop. A second, low-flow coolant loop is connected in series to the high-flow loop. Approximately 10% of the coolant can flow through the low-flow loop. A single pump pressurizes both loops. The low-flow system has the advantage of allowing a relatively small amount of fluid in the low-flow loop to emerge from the high-flow loop and to pass through the radiator. Thus, in the low-flow system, a relatively small amount of hot fluid passes through the radiator. As a result, the low-flow coolant will undergo a greater temperature drop through a given size radiator than would an entire volume of coolant in a conventional coolant system. Thus the cooler fluid exiting the radiator in the low-flow loop can be used to cool the aftercooler to a temperature (almost ambient) which is substantially below that of the engine block or below that of the radiator discharge in a conventional coolant system.
A disadvantage of conventional tuned, low-flow coolant systems which use conventional thermostatically controlled valves in the inability of such a system to respond quickly to transient changes in engine load. For example, in the case of a truck leaving a level road to begin ascending a steep grade, the aftercooler requires additional cooling immediately to maintain engine performance. When the low-flow loop is used to cool the compressed air in the turbocharger aftercooler, the response time to direct coolant through the radiator for more cooling rather than bypassing the radiator can be minutes because the thermostat valve must first be heated by coolant leaving the engine blocking during this transient increase in load on the engine.